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73 nonsmoking indigenous Liberian adults were studied to determine their daily cassava-derived ingestion loads and their subsequent serum thiocyanate levels and thiocyanate-sensitive antibody titers. Given the toxicity of both cyanide and its chief metabolite, thiocyanate, the relationships between these variables were analyzed to identify possible adaptive responses to likely serum thiocyanate overload associated with chronic sublethal cyanide ingestion. Dietary cassava was determined to be the primary source of sublethal cyanide ingestion. The mean daily cassava derived cyanide ingestion level was 0.61 mg/kg body weight. In response, the mean serum thiocyanate level was 22.1 (Jimol/L and the mean thiocyanate-sensitive antibody titer was 1721. Surprisingly, serum thiocyanate levels were not significantly correlated with dietary cyanide intakes. Titers of serum thiocyanate-sensitive antibodies, however, were positively associated with ingested cyanide loads (r = .22, p < 0.05) and negatively correlated with serum thiocyanate levels (r = —.78, p < 0.01). These epidemiologically significant results suggest that among adults chronically consuming 0.6 mg CN ~ and above/kg body weight/day, the resulting thiocyanates may be more rapidly and efficiently cleared from the serum, facilitated in part by immunological mechanisms. In this population, the consumption of a cassava-based diet may act as an agent of natural selection.